The invention relates to an undermattress of the type having a frame which is spanned in a transverse direction by a plurality of laths which are supported on the frame at each end by a respective joint and by a yieldable means for supporting said laths in conjunction with said joints, so as to be yieldable in a direction of loading of the undermattress.
In such an undermattress known from Swiss Patent 388,516 the yieldable means consists of resiliently yieldable support elements on which the laths or battens, which as a rule would be arched in the unloaded state, are supported at both ends, said support elements each being formed by a rubber element which has approximately the form of a triangle with upwardly pointing tip. The corner regions at the base line of this triangular rubber element are provided with bores via which each of said elements is fitted onto two pins anchored on the frame. In the region of the tip the rubber elements have two notches into which the forked ends of the laths engage.
WO 85/02 987 discloses a further undermattress according to the preamble of claim 1 in which the yieldable means consists of an elastic rope which runs over pulleys, the lath ends being tiltably connected to push members of which the other ends carry further pulleys with which the push members bear on the rope led over said first pulleys in guides in the frame of the undermattress.
In this known arrangement a so-called composite effect is achieved in so far as the loading at one point of the rope by a lath of the undermattress influences the rope tension at other points. Thus, in this known undermattress no individual suspension of the laths is provided but on compression of the push members connected to the laths at the rope the tension and thus the spring constant of the rope for the push members of other adjacent laths are influenced in such a manner that said spring constant increases.
In the aforementioned known undermattress the laths are mounted via pins pivotally movable in the longitudinal direction of the frame on the push members so that their angular position to the horizontal direction can adapt itself to the lying of for example a human body on the overmattress disposed above the undermattress.
The known undermattress has however the disadvantage that because the laths mounted at both ends partially on pins or bores due to one-sided body loading are not always horizontally loaded diagonal displacements occur and this results in a transverse positioning of the laths and thus a change of length in the horizontal direction and because there is no length compensation a high frictional engagement can occur between the push members and the frame which can be so pronounced that the push members jam and can no longer fulfill their function.